In a glass sheet heating furnace, relatively long resistor elements are subjected to major temperature variations. Thus, the length variations caused by thermal expansion are also substantial. Another problem is that the resistor element assembly is partly made of ceramics and partly of metal, i.e. it is made of materials whose coefficients of thermal expansion differ a lot from each other. In order to handle these problems and to maintain the length of resistor elements reasonable, the present glass sheet tempering furnaces have grouped the resistor elements in separate arrays of resistors in the lengthwise direction of a furnace. An essential drawback in this solution is that the replacement of resistors requires a lot of labor and long down-times. However, resistors need be replaced quite often, e.g. for setting heating effect (total output, output distribution) and in resistor breakdowns. With present structures and assemblies, resistors are susceptible to damage since it has not been possible to sufficiently eliminate the axial movement between resistance wire coils and ceramic elements. A metal frame having a high coefficient of thermal expansion sets the ceramic elements, whose co-efficient of thermal expansion is lower, in uncontrolable axial movement which in time leads to build-up of resistance wire coils.
In terms of the design and operation of a furnace it would be preferable to make the overhead resistors suspended from the ceiling of a furnace and the bottom resistors mounted on the floor of a furnace mutually exchangeable. With present resistor element assemblies, they are not exchangeable due to the different method of supporting and mounting the resistor elements.